This invention relates to composite porous media and, more particularly, to composite porous media for filtering gases and liquids.
Porous media are used in a wide range of industrial applications for filtering and dispersing gases and liquids. Typical examples of such uses include particle capture, flow restriction, sound attenuating, gas/liquid contacting, wicks, spargers, and atomizers. In the electronics industry, high efficiency filters are widely used to remove small particles from a variety of process streams. Sintered metal filters useful for this purpose and capable of removing more than 99.9999999% of particles having the most penetrating particle size, e.g., about 0.1 micrometer, from a process gas flow (i.e., having a log reduction value or xe2x80x9cLRVxe2x80x9d greater than 9) are illustrated in U.S. Pat. Nos. 5,114,447 and 5,487,771, and in co-pending applications Ser. Nos. 08/895,604 and 08/895,605, now U.S. Pat. Nos. 5,937,263 and 5,917,066, all of which are here incorporated by reference. A wide range of sintered metal media for use in the these various fields and applications, including high efficiently filters for the electronics industry, are available commercially from Mott Corporation, the assignee of this patent. Sintered porous ceramic media and polymeric membrane filters are also avaiable commercially.
As described in the above referenced patents and applications, and as is well-known in the art, porous metal media are typically made by pressing or molding metal or metal-alloy powders of specified characteristics into a desired shape, e.g., a sheet, tube or cup. The shaped body is then sintered at high temperature to provide a porous element or media. Sintered porous ceramic media are made by similar known procedures.
The exact characteristics of a porous metal or other media are highly dependent on a number of factors, including the particular powder used, the green density, the sintering conditions employed, and the configuration of the media. Depending on the application, important physical characteristics of the media may include its resistance to corrosion (e.g., from reaction with a wide range of process gases and liquids), mechanical strength, and the ability to withstand high temperatures. A filter, for example, should provide a relatively high rate of fluid flow at minimum pressure drop, and must be capable of removing any particulate matter that could cause contamination in the downstream manufacturing process.
As is known in the art, sintered powder metal media are generally capable of providing the desired corrosion and high temperature resistance. However, and as also is well-known, the relatively high porosity needed to provide the desired flow at a low pressure drop often comes at the cost of low mechanical strength and a decrease in efficiency. The lack of strength of conventional porous media is most serious in the xe2x80x9cgreenxe2x80x9d form; poor handleability of xe2x80x9cgreenxe2x80x9d porous structures is a major concern in manufacturing procedures particularly for large dimension tubes and sheets.
Fine filamentary nickel powders, such as those produced by the carbonyl nickel process and sold by INCO, are being used commercially to create fine porous media.
However, the xe2x80x9cgreenxe2x80x9d shapes made from these fine powders are relatively weak, and this is particularly so when the powders are more spherical than filamentary in shape. Moreover, fine powders have a high surface area and are highly active during the sintering process; thus, 10% to 15% shrinkage of the molded xe2x80x9cgreenxe2x80x9d shapes occurs when they are sintered at 1300-1700 degrees F., in either a protective atmosphere (such as hydrogen) or in vacuum, is common. Additionally, because these powders are typically sintered at low temperatures (e.g., 1300 deg. F.) and for relatively short times (e.g., 10 to 15 minutes) to maintain high porosity, even the final sintered structure is relatively weak since both the time and the temperature are less than that necessary to form strong sintered bonds. Thus, although such porous media are satisfactory for relatively small structures, severe limitations in the processing and mechanical properties of porous sintered metal structures have made it difficult to make larger structures, such as large sheets, continuous strips or tubes of substantial diameter or length. Porous ceramic media having pore sizes of the same order of magnitude as those of sintered metal powder filters have been made using fine oxide particles, e.g., of 1 to 5 micrometer size, but these ceramic elements tend to be considerable more dense, e.g., to have 55% to 75% of theoretical density.
There remains a need for a porous media having the highly desirable flow, heat and corrosion resistance characteristics of the best sintered powdered metal or ceramic media, but that is stronger, can be formed into larger structures, and that can provide greater overall flow at a low pressure drop. There is a particular need for structures which have a high xe2x80x9cgreen strengthxe2x80x9d, to facilitate handling and transfer during the manufacturing process.
The present invention provides a composite porous media that can be used for either gas or liquid flow, that is strong (both in its final and green form) and efficient, and that can readily be formed in or into a wide range of different shapes and configurations. In particular, the invention features a porous media that is a composite of a metal, ceramic or other open-pore foam (i.e., a reticulated, inter-cellular structure in which the interior cells are interconnected to provide a multiplicity of pores passing through the volume of the structure, the walls of the cells themselves being substantially continuous and non-porous, and the volume of the cells relative to that of the material forming the cell walls being such that the overall density of the reticulated cellular structure is less than about 30 or 35 percent theoretical density) and sintered powder in the pores of the foam. The thickness, density, porosity and filtration characteristics of the final composite porous media can be varied to conform with what is required by the intended use. Typically the size of the pores formed by the foam is in the range of a few hundred to a few thousand micrometers, while the sintered powder in the foam pores is much smaller than, and forms a pore structure that is much finer (e.g., at least an order of magnitude and often two or three orders of magnitude smaller) than, the pores of the foam. Typical powder dimensions are in the range of less than 1 to 50 micrometers.
In some preferred embodiments, the pores of a foam having a density in the range of about 5% to about 15% theoretical density, and about 10 to about 150 pores per linear inch, are impregnated with a powder that is orders of magnitude smaller than the pores of the foam per se and that has anti-corrosive properties at least equal to those of the foam. The foam, which typically has a thickness in the range of about 0.020 to about 0.250 inches, may be in the form of a sheet, or it may be manufactured in the shape of a tube or other regular shape of engineering interest. Because of nickel""s corrosion-resistive properties, nickel (or nickel alloy) foams and powders are often preferred, and the nickel powder is typically filamentary.
The composite media typically has a density of about 20-70% of theoretical density and the pores formed by the powder are on the order of I micrometer or smaller in size. For high efficiency filtration, the composite media may include a layer of sintered powder overlying a foam (with powder in the pores of the foam) core and, when a ductile (e.g., a metal) foam is employed, the foam will have been compressed to a fraction of its original thickness so that the resulting composite media has a total thickness that is substantially the same as (or thinner than) the original thickness of the foam per se. For liquid filtration, a thinner, e.g., about 0.010 inches, and more porous composite media is often desirable.
In one preferred practice, the composite media is made by filling the pores of a foam sheet with either dry powder or a powder slurry, if the foam is ductile (e.g., metal) cold pressing or rolling the filled structure to produce a xe2x80x9cgreenxe2x80x9d form of the desired xe2x80x9cgreenxe2x80x9d density, and then sintering. In a second practice, a cylindrical ductile metal foam sheet is placed coaxially around a mandrel and then is in turn surrounded by an elastomeric mold (or, alternatively, a cylindrical form sheet is placed around a hollow cylindrical elastomeric mold and in turn is surrounded by a cylindrical exterior mandrel), the annular spaces (i) between the mandrel and the foam cylinder and/or (ii) between the foam cylinder and the mold are filled with powder to impregnate the pores of the foam, pressure is applied to the mold to compress and densify the powder and foam, and the compressed xe2x80x9cgreenxe2x80x9d powder-filled foam is then sintered. In either practice, a layer or xe2x80x9cskinxe2x80x9d of sintered powder having a thickness that may be a substantial percentage (e.g., up to about 70%) of the overall thickness of the composite media may be provided on at least one surface of the foam. In both processes, the forming/pressing compresses the thickness (e.g., wall thickness) of a ductile foam, and increases the density of the green structure. other objects, features and advantages will become apparent from the following detailed description of embodiments of the invention, taken together with the attached drawings.